Lord Slashcat said:You're right! Flint size is generally determined by the width of the frizzen. There is a little more to it of course. The length of the flint must be such that it is not resting on the frizzen while the lock is at half-cock with the frizzen closed. This is how the lock will be before you are ready to fire or while you are hunting. Some older locks in particular use a shorter profile flint in order to achieve this. It's okay if the flint is slightly narrower than the frizzen but not if it is wider. Usually,this is because the flint may strike the side of the barrel near the toch hole and shatter if its too wide.
If you don't have the right length flint,you can often shorten it by knapping either the front or the rear of the flint with a hammer. A really nifty way is to make a "U" shaped notch at the rear of the flint to correspond with the screw shank at the rear of the jaws. This preserves the cutting edge of the flint and will yield more shots.
I currently buy most of my flints from trackofthewolf.com and they often post a reccomended flint size for each lock,especially in their gun listings.
Flints are held in the jaws of the cock with a piece of deer skin wrapped around them to protect them from shattering when the cock jaws are tightened and from impact. Another method of mounting a flint is to take a lead ball and pound it flat on an anvil (or the sidewalk) with a hammer and fold it over the flint before mounting. This works really neat!